1. Field of the Invention
The present invention relates generally to a system for calibrating the output plane of an optical correlator for positional information and nonlinearities therein. More particularly, the subject invention pertains to such a calibration system which enables appropriate positional calibration targets to be generated and utilized with a high degree of precision of location between an input image and the output plane of the optical correlator.
2. Discussion of the Prior Art
An optical correlation system is disclosed in U.S. patent application Ser. No. 814,209, filed Dec. 27, 1985, now abandoned and refiled in U.S. patent application Ser. No. 236,519, relative to which the output plane correlation system of the present invention was developed. The optical correlation system disclosed therein optically compares an input image with optical information stored in multiple matched filters to provide identification and aspect information about the input image. In one disclosed embodiment, the input image is directed onto a spatial light modulator to spatially modulate a coherent beam of radiation. The spatially modulated radiation beam is directed onto a multiple holographic lens which performs a multiple number of Fourier transformations thereon to obtain an array of a multiple set of Fourier transforms of the spatially modulated radiation beam. A corresponding array of matched filters has the array of Fourier transforms incident thereon, with each matched filter comprising a Fourier transform hologram of an aspect view of an object of interest. Each matched filter passes an optical correlation signal in dependence upon the degree of correlation of the Fourier transform of the spatially modulated radiation beam with the Fourier transform hologram recorded thereon. An inverse Fourier transform lens receives the optical correlation outputs of the array of matched filters, and performs an inverse Fourier transformation on each optical correlation output, which is directed to an output correlation plane. A detector at the output correlation plane then detects the optical correlation outputs, and a processing circuit determines identification and aspect information about the input image.
One problem with this type of optical correlator is that of calibrating with precision the output correlation plane for positional information and nonlinearities therein relative to the position of input images in the input image plane. Such calibration should also enable a determination of the output field nonlinearities to arrive at a compromise on the position of an input image frame or the position of one input target if the output field is not linear.
Suemoto and Ohara disclose in "Accurate Position Detection of Targets in Noise Image", Optics Communications, Vol. 54, July 15, 1985, the use of circular rings placed in an image frame, and one placed just outside of the image frame, to make a matched filter for the circles, and then relate the correlation plane spots to the input plane spacings for measurements.
This publication illustrates an input plane image in which rings 1-3 are placed in the frame and a ring 0 is placed outside the normal frame. The various distances 0-1, 1-3, 2-3,....are measured with precision A matched filter is made of a ring placed on axis. Upon playing the input frame back, the correlation plane shows four bright spots A-D, corresponding to the rings, 0-3. Measurement of the distance A-B (=d) corresponds to the distance between the circles 0-1, and careful measurement of either circle 0 or circle 1 to the frame edge, enables the edge to be properly located in the correlation plane image. This approach is utilized because on playback, the sharp frame edge gives an "edged effect" in the correlation plane consisting of an oscillatory variation in intensity where the edge is, and an outward shift in edge location. These effects give rise to edge uncertainty, an important anomaly when aerial reconnaissance imagery is being processed and reference positions are desired.
Suemoto and Ohara do not indicate the degree of spatial frequency cut-off, and so it is not clear that the location of the edge is obtained to the highest degree possible. Accuracy that is achieved is given as .+-.1.8%. Greater accuracy can possibly be achieved with higher pass matched filters as the correlation spikes tend to get smaller and sharper. Thresholding to a higher level should also increase accuracy. The author does not indicate an awareness of these factors nor an ability to calibrate without changing the matched filter, which operation always alters the alignment in the optical correlator, which presents alignment problems for subsequent usage of the correlator.
In summary, this approach is effective to some extent for determining spatial distances, but introduces concurrent alignment problems. Also, several constraints limit the use of this prior art technique. Most obvious is that in many applications of an optical correlator as described herein the input imagery has highly colored noise and nonsymmetrical targets. Other commercial applications such as robotics provide the opportunity for a well controlled background.
Moreover, it is not always possible or desirable to change the matched filter in the system from a calibrating target MF to a MF required for targets to be anticipated in actual industrial or commercial usage of the optical correlator as this normally requires recalibration of the optical correlator. This is most pertinent when the optical correlator system uses a MHL. The requirements for the alignment of a large MHL array are manageable, but the aligned components of the system should not be moved around any more than necessary. Thus a calibration system for the present invention should use a working matched filter, i.e. one for an image of an object anticipated to be encountered in actual industrial or commercial usage of the optical correlator.
Next, full frame imagery is most frequently encountered in commercial and industrial applications and thus, a portion of the frame cannot use a calibration signal extending beyond the frame. This means, for example, that a 35mm format size (35mm .times.24mm) is the format size for which the system, including optics, is designed. Anything beyond the format is excluded.
Finally, the calibration target could be one of the targets included in the memory bank, which ties in with the second constraint. In addition, the definitions for distance would be established by the signals in the correlation plane one must deal with.